COMPRESSIBILITY OF METALS. 223 



charge e (dimensions of e 2 are MLT -2 ) and 8, the distance of separation 

 of atomic centers (L). The required combination is at once found to 

 be 8 i /c-. The very fact that it is possible to build up a combination of 

 these two quantities of the right dimensions is presumptive evidence 

 of the correctness of our general considerations, because in general it 

 would require three (instead of two) quantities to give in combination 

 the dimensions of any one arbitrarily given quantity. This dimen- 

 sional argument suggests, therefore, that compressibility should be of 

 the order of magnitude of 8 i /e 2 . Now assuming simple cubic structure, 

 and expressing 5 in terms of the quantities in terms of which it is de- 

 termined in practise, that is in terms of atmoic weight, density, and 

 the mass of the hydrogen atom, we find that the compressibility is of 



(Mass of H atom) t ; /At.Wt.\f 



the order of magnitude of : X ( v^ r - ) , or sub- 



e 2 \Density/ 



stituting numerical values for e and the mass of the hydrogen atom, 



. /At. Wt.\! 

 Compressibility is of order of 8.6 X 10 _H I — — I . 



Here of course, compressibility is in absolute units, that is, the unit of 



pressure is 1 dyne/cm 2 . 



In table V is given the compressibility in absolute units divided by 



/At. Wt.\t 

 10" 14 ( — '- — 7— ) , for most of the metals measured above. It is seen 

 \Density/ 



that the numbers are of the order of magnitude of 8.6, as our argument 



suggests. 



Although the argument is crude, it seems to me that the result is of 

 great significance, because it suggests that in the metal, as well as in 

 salts, the atoms become charged by losing valence electrons, and the 

 metallic structure is held together by the forces between positively 

 charged residues of the atoms and the lost electrons. It is furthermore 

 to be presumed that the lost electrons take some definite position in 

 the crystal lattice, as for instance, the measurements of Hull 20 make 

 so probable in the case of calcium. 



To make the next step in refinement in the computation, we must 

 take account of the exact arrangement of the atoms in the lattice, and 

 must also know the law of repulsion. As far as the arrangement of 

 the atoms in the crystal structure goes, the results of X-ray analysis 

 indicate very definitely in at least some cases what we must expect. 

 Thus Hull 20 has found in the case of metallic calcium that the Ca 

 atoms have exactly the same arrangement as the Ca atoms in CaF2. 



